It has been recognized that niobium monoxide (NbO) has some unusual electrical properties that make it well-suited for the manufacture of electronic capacitors. It is of much lower flammability than equivalent tantalum powders, is less costly than tantalum, and has much larger potential supply than tantalum. However, niobium monoxide capacitor powders require high levels of purity, with not only foreign elements such as iron and copper being deleterious, but other forms of niobium such as niobium metal, niobium dioxide (NbO2), niobium trioxide (Nb2O3) and niobium pentoxide (Nb2O5) being harmful. In order to be useful in a valve application, the niobium monoxide must be in a finely divided form, i.e., fine powder or, preferably, agglomerates formed from small particles, such small particles typically about 1-2 microns in diameter or finer. In order to meet these requirements, the electronics industry has produced niobium monoxide by reacting agglomerated and sintered niobium pentoxide or niobium dioxide (optionally pre-reduced from the pentoxide) with a metallic reducing agent under conditions in which the niobium oxides remain in the solid state. This allows the particle morphology of the original agglomerated oxide to be preserved in the niobium monoxide. In one embodiment of this process, niobium pentoxide is reacted at temperatures of approximately 1000° C. with finely-divided metallic niobium, in such stoichiometric proportions as to produce primarily niobium monoxide. In another embodiment, the niobium pentoxide or niobium dioxide is reacted with gaseous magnesium, again at temperatures of approximately 1000° C. This results in a spongy, highly porous niobium monoxide-magnesium oxide mixture. After leaching the magnesium oxide, the resultant product is a porous, high-surface area agglomerated mass of niobium monoxide.
Because of the low processing temperatures used in these methods of producing niobium monoxide, there is virtually no opportunity to remove any impurities in either the niobium oxide or the reducing agent feedstocks. Moreover, impurities on the surface of the feedstock particles remain on the surface through the solid-state processing, resulting in potentially detrimental concentrations of these impurities on the surface of the NbO particles. The electronic characteristics of capacitors produced from such surface-contaminated particles may be seriously degraded. The purity requirements of the niobium monoxide dictate the purity required of the feedstock. The surface area requirements of the product niobium monoxide dictate the particle size distribution and morphology of the niobium pent-or-di-oxide and niobium metal needed for the process. These requirements severely limit the availability of suitable raw materials. Further, because the reactions occur in the solid state, the reactions are sluggish and often do not go to completion. The product contains some higher oxides of niobium, and often some niobium metal.
Thus, an object of the present invention is to produce niobium monoxide (NbO) powder of high purity and sufficient surface area to meet the requirements of NbO capacitors without the constraints of raw materials purity and particle size imposed by solid-state processes, and the use of such powders in the production of capacitors. The present invention also can be used to produce high-purity niobium dioxide, and to produce large, (non-particulate) non-porous objects of both niobium monoxide and niobium dioxide. The powders produced from such objects are non-porous and angular in shape.